Method of processing steel material having high austenitic grain-coarsening temperature

ABSTRACT

A method of processing steel material having a high austenitic grain-coarsening temperature and an abnormal grain growth curve of cold worked austenitic grain, which comprises heating said steel material at a temperature of from the ferrite recrystallization temperature to the A1 transformation point for more than five minutes to cause recrystallization. This technique prevents the formation of mixed grains upon austenitizing and thereby preserves the mechanical properties of the steel material.

United States Patent Suzuki et al.

METHOD OF PROCESSING STEEL MATERIAL HAVING HIGH AUSTENITIC GRAIN-COARSENING TEMPERATURE Inventors: Akira Suzuki, Kobe; Shushi Kinoshita; Takeshi Ueda, both of Akashi, all of Japan Assignee: Kobe Steel, Limited, Japan Filed: July 18, 1973 Appl. No.: 380,460

Related US. Application Data Continuation-in-part of Ser. No. 134,332, April 15, 1971, Pat. No. 3,788,903.

Foreign Application Priority Data Apr. 15, 1970 Japan 45-32078 US. Cl. 148/12 R, 148/12 F, l48/12.l Int. Cl. C211] 7/02, C21d 7/14 Field of Search 148/12, 12.1

[451 Feb. 11,1975

[56] References Cited UNITED STATES PATENTS 3,711,342 l/1973 Cullen et a1. 148/12 Primary ExaminerW. Stallard Attorney, Agent, or Firm-Oblon, Fisher, Spivak, McClelland & Maier [57] ABSTRACT 8 Claims, 4 Drawing Figures PATENTEI] FEB] 1 I875 FIG. 18

FIG. 1A

FIG. 28

FIG. 2A

METHOD OF PROCESSING STEEL MATERIAL HAVING HIGH AUSTENITIC GRAIN-COARSENING TEMPERATURE RELATIONSHIP TO RELATED APPLICATIONS BACKGROUND OF THE INVENTION 1. Field Of The Invention This invention relates to a method of processing steel material having a high austenitic grain-coarsening temperature, and more particularly to a method of processing steel material without the danger of producing a mixed grain structure upon heat treatment in the austenitic region following cold working.

2. Description Of The Prior Art If carbon steel and alloy steel materials containing elements having grain refining action, such as aluminum, titanium, niobium, etc., are heated in their austenitic region, they show an abnormal grain growth curve. When steel containing a grain refining element is cold worked at a temperature lower than the ferritic recrystallization temperature, the temperature at which austenitic grains on subsequent austenitization will start to abnormally grow, will be depressed, and will result in a mixed grain structure.

The mixed grained steels are usually considered to be of inferior quality, and, when such steels are heat treated under severe austenitizing conditions, such as carburizing in the austenitic region following cold working, difficulties with toughness, strain and distortion can result.

Hcretofore, it has been found that heating the steel at a temperature above l,000C. after cold working, will reduce the dangers of producing a mixed grain structure since the austenitic grain-coarsening temperature of the cold worked steel will be increased. However, such high temperature heat treatment can cause serious scale formation which can result in severe dimensional inaccuracy and can impair the smooth surface created by cold forming. Moreover, such high temperature heat treatment is not economically desirable from a commercial cost standpoint.

SUMMARY OF THE INVENTION Accordingly, it is one object of the present invention to provide a method of processing steel which improves the depressed austenitic grain-coarsening temperature caused by cold working, and minimizes the danger of formation of mixed austenitic grain structure upon subsequent austenitizing.

It is another object of this invention to prevent th deterioration of toughness, strain and distortion of steel material due to the presence of a mixed austenitic grain structure.

Another object of the present invention is to provide a method of processing a steel material without losing the desirable attributes of cold working, including its excellent dimensional accuracy, smooth surface skin, etc.

A still further object of the present invention is to provide a method of processing a steel material which does not create mixed grains during carburizing treat ment following cold working in the preparation of machine elements.

These and other objects have now herein been attained and the austenitic grain-coarsening temperature increased by recrystallizing the cold worked ferritic structure at a temperature within the ferritic recrystallization region, below A, transformation temperature, before the austenitizing heat treatment. It is believed that this technique reduces the driving force of the austenitic grain growth upon heat treatment of the steel material.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the invention will be readily obtained with the aid of the following description in conjunction with the accompanying Tables and photographs.

FIGS. 1(a) and 2(a) are photographs (X) showing the preferable fine grain structure obtained after austenitizing the steel material at 950C. by the method of this invention; and

FIGS. 1(b) and 2(b) are photographs (X100) showing mixed grain structure appearing after austenitizing the same steel material at 950C. by the conventional method.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Any steel material which is characterized by abnormal austenitic grain growth, i.e., any steel material in which the austenitic grains start to grow suddenly at a temperature in the austenitic region, can be treated according to the present invention. For instance, this technique can be successfully applied to carbon steels and alloy steels containing one or more grain refining elements, such as aluminum, titanium, niobium, zirconium, vanadium, tantalum, rare earth elements, etc. which precipitate as nitrides, carbides and oxides.

The steel material is prepared for cold working, after hot working and/or heat treatment, etc., and is formed to a desired shape by rolling, forging, extruding, etc., at a temperature below the ferritic recrystallization temperature region. Mechanical working, such as machining, may be used as the cold working.

The cold worked steel material has a lower austenitic grain-coarsening temperature than non-cold worked steel material as aforementioned. This steel material is then recrystallized at a temperature within the ferritic recrystallization region so as to raise the austenitic grain-coarsening temperature and to thereby cause the cold worked ferrite structure of the steel material to substantially recrystallize in the ferritic region. To accomplish this, the steel material must be heated to at least the temperature within the ferritic recrystallization range, but below the A, transformation temperature. The specific temperature range will, of course, depend upon the type of steel and the cold working conditions, but the lower limit is usually about 400 500C., while the upper limit is about 700 800C.

If the temperature of the heat treatment is lower than the lower limit, the deformed ferritic structure will not substantially recrystallize, while if the temperature of the heat treatment is higher than the upper limit, the deformed ferritic structure will not be recrystallized and it will result in lower austenitic grain-coarsening temperature with its consequent risk of mixed grain structure formation on subsequent austenitizing. In general, within the desired temperature range, the

lower the heating temperature, the longer the period of heating time required. The shortest time period is 5 minutes, but periods of longer than 30 minutes are preferred. When the heat time period is less than 5 minutes, sufficient recrystallization will not occur.

One good method for heat treating is to hold the steel material at a constant temperature within the ferritic recrystallization region, and thereafter to cool it. However, the same effect may be expected by cyclically heating and cooling within the ferritic recrystallization region at a sufficient cyclic rate.

The heat treatment in the ferritic recrystallization region according to this invention will be effective regardless of whether the austenitic grain growth inhibitors are nitrides, carbides or oxides and regardless of whether they have already precipitated before or are in the process of precipitating before the ferritic recrystallization treatment. It is also effective, regardless of the state of the precipitation, and regardless of the amount, number, distribution and/or morphology of the particles. However, the positive utilization or progression of the precipitation during the ferritic recrystallization treatment will further raise the austenitic graincoarsening temperature, since it results in finer austenitic grain growth inhibitor particle size Another characteristic feature of this invention, therefore, is that the combination of increased inhibiting force against graincoarsening and reduced grain growth driving force on subsequent austenization will raise the austenitic graincoarsening temperature. The increased inhibiting force is derived from the precipitation treatment of the supersaturated steel materials in the ferritic region, and the reduced grain growth driving force is a result of the ferritic recrystallization treatment following cold workmg.

Since it is necessary to dissociate the nitrides and carbides into a solid solution, during the solution treatment of steel materials, it is necessary to heat the steel to a temperature of at least l,000C. for an excess of 5 minutes. if the heating is below l,000C. or for shorter periods than 5 minutes, the dissociation of the precipitate will not be sufficient and the sufficient hibitor on the succeeding aus'tenization. This increases the austenitic grain-coarsening temperature. Heating of the supersaturated steel material is most preferable in the temperature range of 500C. to the A, temperature for more 5 min. The precipitate forms as fine particles in the ferritic structure and the rate of precipitation will depend upon temperature. Below 500C., the rate of the precipitation is so slow that there is substantially no precipitation progress. On the other hand, the precipitatiorf treatment in the austenitic region results in coarser precipitate particles which are not desirable. l-leretofore, it was quite common to apply a normalizing treatment prior to cold working. Normalizing should not be conducted as the heat treatment, however, before cold working in this invention.

Heat treatment of the steel material in the higher ferritic temperature region before cold working reduces the deformation resistance upon cold working, and raises the critical working rate above that at which cold working cracks are generated. If more severe cold working performance is required, this treatment may amounts of fine particles necessary to prevent austenitic grain-coarsening at lower austenitic temperatures a on subsequent austenitization, may not be obtained during the ferritic precipitation treatment.

The cooling rate after solution treatment should be not less than 5c./min. in the temperature range of 900 to 500C. Cooling rates of less than 5C./min. give rise to the precipitation of a considerable amount of coarse precipitate, which is less effective in the prevention of .austenitic grain-coarsening on subsequent austenitization, and gives rise to a reduced amount of finely precipitated particles in the following precipitation treatment. Finely precipitated particles are more effective in inhibiting the austenitic grain-coarsening.

Hot working, such as rolling, forging and extruding may follow the solution treatment to impart cold work be combined with a spheroidizing annealing in order to improve cold workability.

Austenitizing by intermediate heat treatment, such as annealing which may be carried out between cold working processes, should also be avoided, in order to inhibit austenitic grain-coarsening.

As will be clearly described in the embodiment of the present invention, when the techniques of this invention are applied to cold worked steel materials, the austenitic grain-coarsening temperature will be raised, and thus the dangers of mixed grain structure formation upon austenitization with its consequent deterioration of quality is minimized. Since the method of this invention does not necessitate high temperature treatment,

' compared with conventional methods for achieving (Unit: wt.%

Contents Type of Sol Steel C Si Mn P S Cr Mo Ti Nb A1 TABLE 1: Chemical. Composition Ol 'lcst Materials -Continued (Unit: \Vt."/

Contents Type of S01 Steel C Si Mn P S Cr Mo Ti Nb Al In Table 1, steel A contains aluminum needed to nor- Room Temperature in Table 2 means that austenitizing was conducted directly after cold working. In Table 2, the increase of austenitic grain-coarsening temperature of the steel material is recognized in the ferritic recrystallization temperatures at and above 450C.

Table 3 shows the austenitic grain-coarsening temperatures in relation to the heating time at 700C. for ferritic recrystallization, for steel B produced in the same process as that for Table 2.

TABLE 3 Holding Temperature for Ferritic Recrystallization And Austenitic Grain-Coarsening Temperature 700 800 Type Holding Holding of Steel Time 5 20 60 300 Time 5 20 60 300 A 925 950 950 975 900 925 925 950 B 950 950 975 1000 925 950 950 950 F 950 975 I000 I000 925 950 950 950 NOTE:

Steel A: hot working normalizing cold working Steel B: hot working annealing cold working Steel F: hot working spheroidizing cold working and F, produced by the following process:

1,100C. X1 hr. solution treatment hot work air cooling (average cooling rate: 20C/min.) ---'50% cold working.

As is clear from Table 3, the increase of the austenitic grain-coarsening temperature is evident for holding times of 5 minutes or more at 700C.

The uniform and fine microstructure was obtained by TABLE 2 Holding Temperature For Ferritic Recrystallization And Austenitic Grain-Coarsening Temperature Holding Temp.(C) Room Holding Temp. After Cold Working Type of Transformation Temp. 400 450 500 600 650 700 725 750 775 800 825 850 900 Steel Temp. (C)

A 730 840 900 900 9l0 925 925 925 950 950 950 950 925 925 900 900 B 740 845 925 925 940 950 950 975 975 1000 975 975 950 950 925 900 F 740 845 925 950 950 975 1000 I000 I000 950 950 925 925 NOTE:

Steel A: hot working normalizing cold working Steel 8: hot working annealing cold working Steel F: but working spheoridizing cold working ll00C. X 1 hr. solution treatment 50% hot working air cooling (average cooling rate 20 C/min) 50% cold working.

7 8 the combined heat treatment of l) holding at 700C. TABLE S-Continued for 1 hour followed by (2) air cooling and austenitizing at 950C. (followed by quenching) as shown in FIG. r ating Processes and Austenitic Grain- 1(a) for steel B which was produced by the process in- 5 Coarsenmg Temperatures dicated in the affix of Table 2. On the other hand, the (Type of Saw-1; E) I mixed grain structure was obtained after austenitizing Type of 32:? at 950C without heating at the ferritic recrystallization Steels Process (C-J temperature of 700C. 100C X 1 h l A uniform and line structure, similar to that in FIG. 10 hot work'ing 8 i3?';{ ,{3* 2(a), was obtained for steel C by the same treatment as g sfor FIG. 1(a), as shown in FIG. 2(a). On the other .23; 1 hr 30mm treatmenw 925 hand, a mixed grain structure was obtained for steel C hot w0rking* 500C. X l hr.AC treated in the same way as for FIG. l(b), as shown in z zi ggl 700 950 FIG. 2(1)). *l 100C. X 1 hr. solutiontreatment- Table 4 shows the comparison between the austenitic f ggg 'g g jgz gg jgdg grain-coarsening temperatures of steel D treated by the X 10 min.AC 925 r e 4 process of the 1nvent1on: h A a 2&5? B533 0??? fig solut1on treatment coolmg cold workmg 50% cold w0rking- 700C.

ferritic recr stallization X l hr-AC 975 y 1 100C. X l hr.solution treatment or hot working* 700C. X l hr.AC

solution treatment cooling AlN fine precipi- 283 :3 ig 88:gg? wig 1000 r Q 0 CO WOT In a I. tatron treatment cold working A ferrmc rerr c X 1 h solgufion treatmemfi crystalllzatlon, and by the convent1onal treatment: hot orking 1 I 700C. X l hr.AC so ut1on treatment coolmg normahzmg 50% cold i g ,7oooc X 1 MAC 925 cold working. Conven- *1 100C. X 1 hr. solution treatment tional hot working* 900C. X l hr.AC TABLE 4 Treat- 50% cold working 875 ment Treating PIOCCsE And Austcnitic 3O indicates mean cooling speed after hot working 8(./min. Gram-Coarsenmg Temperature indicates mean cooling speed after hot working 8"(./min.

Type of Steel: D

Coarsening Table 6 shows the austenmc gram-coarsenmg temfi' g perature of the steels F and G treated by the respective processes. This *1 lO0C. X 1 hr. solution treatment lnven- 700C. X l hr.AC 50% cold working tion *700C. X l hr. AC 975 TABLE 6 1 100C. X 1 hr. solution treatment I I 900C X l hr-AC 50% cold worklng 950 Treating Processes and Austenitic Grain- Convenf 1 100C. X l hr. 900C. X l hr.AC 40 coarsening Temperatures tional 50% cold working 900 Treat (Types ofSteels: F and G) ment 1 Coarsening yp of Temp. indicates mean cooling speed after solution treatment 8C. min. I Steels Process (C.)

' r' *110o=c. 1r 1 tetXlhr. Table 5 shows the companson between the austenitic v hot jgi'i? 6 618} MAC 6 rain-coarsenin tem erature of the steel E treated b tion cold working- 700C. X g p y 1 h AC 1000 the process of the invention:

solution treatment hot working cooling "100C I t X 1 h 'AlN fine precipitation treatment cold working 50 y i figfi'lfg sfg il flg (intermediate heat treatment cold work- +700C. X I hr.AC 975 q ferrmc recrystamzat'? or F 1100"C. solution treatment X 1 hr. solutlon treatment hot working -r slower cool- .-+hot workin *?900C. X 1 hr.AC

ing AlN fine precipitation treatment (slightly) x 1 MAC 950 cold working ferritic recrystallization, and *1100c. solution treatment X 1 hr.

hot working* 700C. X l hr.AC convemlonal process 5o% cold working -+7o0c. x 1 hr.AC solut1on treatment hot workmg cooling m working 700c. x 1 hr.AC 1000 normahzmg cold workmg' Conven- *ll00C. solution treatment X l hr.

tional hot working* TABLE 5 6O 900c. x 1 hr.AC- 50% cold working 925 And Ausenmc Grain This *1100c. x 1 hr. solution treatment Coarsenmg Temperaure lnvenhot working* 700C. X

tion I hr.AC" 50% cold working yp of el: 700C. X I hr.AC 950 Coarsenmg 5 Temp. 6 0 Process CI) Conven- *1 lO0C. solution treatmcent X lhhr. tional hot working* 900 X l r.AC Thls l lO0C. X 1 hr. solutlon treatment lnvenher working* 500C. x 50 hrs.AC +50% 875 tion 50% cold working-r 500C.

X 30 hrs AC 925 indicates mean cooling speed after hot working 8C.]min.

Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the invention. Accordingly,

What is claimed as new and intended to be covered by Letters Patent is:

l. A method of processing steel material which is characterized by an abnormal grain growth curve of austenitic grain after cold working and which has a high grain coarsening temperature which comprises:

heating said cold worked steel material to a temperature of between the ferrite recrystallization temperature and the A, transformation temperature for a period of at least minutes so as to effect recrystallization of the ferritic structure which had become deformed during cold working, and thereby raising the austenitic grain coarsening temperature,

further heating said steel material to a temperature within the austenitic region of from the A, transformation temperature to the austenitic grain coarsening temperature to effect austenitizing to form a non-mixed grain structure.

2. A method of processing steel material which contains a grain refining element which is characterized by an abnormal grain growth curve of austenitic grain after cold working and which has a high grain coarsening temperature which comprises:

heating said cold worked steel to a temperature of between the ferrite recrystallization temperature and the A, transformation temperature for a period of at least 5 minutes so as to effect recrystallization of the ferritic structure which had become deformed during cold working, and thereby raising the austenitic grain coarsening temperature, and to precipitate said grain refining elements in the form of nitride, carbide and/or oxide,

further heating said steel material to a temperature within the austenitic region of from the A transformation temperature to the austenitic grain coarsening temperature to effect austenitizing to form a non-mixed grain structure.

3. The method of claim 2, wherein said steel material is hot worked subsequent to the first heating stop and prior to cooling in the 900 500C range.

4. The method of claim 2, wherein cold working is sequentially cycled with intermediate heat treating, be-

8. The method of claim 1, wherein after recrystallization said steel material is subjected to carburization treatment in the austenitic region. 

1. A METHOD OF PROCESSING STEEL MATERIAL WHICH IS CHARACTERIZED BY AN ABNORMAL GRAIN GROWTH CURVE OF AUSTENITIC GRAIN AFTER COLD WORKING AND WHICH HAS A HIGH GRAIN COARSENING TEMPERATURE WHICH COMPRISES: HEATING SAID COLD WORKING STEEL MATERIAL TO A TEMPERATURE OF BETWEEN THE FERRITE RECRYSTALLIZATION TEMPERATURE AND THE A1 TRANSFORMATION TEMPERATURE FOR A PERIOD OF AT LEAST 5 MINUTES SO AS TO EFFECT RECRYSTALLIZATION OF THE FERRITIC STRUCTURE WHICH HAD BECOME DEFINED DURING COLD WORKING, AND THEREBY RESING THE AUSTENITIC GRAIN COARSENING TEMPERATURE, FURTHER HEATING SAID STEEL MATERIAL TO A TEMPERATURE WITHIN THE AUSTENITIC REGION OF FROM THE A1 TRANSFORMATION TEMPERATURE TO THE AUSTENITIC GRAIN COARSENING TEMPERATURE TO EFFECT AUSTENITIZING TO FORM A NON-MIXED GRAIN STRUCTURE.
 2. A method of processing steel material which contains a grain refining element which is characterized by an abnormal grain growth curve of austenitic grain after cold working and which has a high grain coarsening temperature which comprises: heating said cold worked steel to a temperature of between the ferrite recrystallization temperature and the A1 transformation temperature for a period of at least 5 minutes so as to effect recrystallization of the ferritic structure which had become deformed during cold working, and thereby raising the austenitic grain coarsening temperature, and to precipitate said grain refining elements in the form of nitride, carbide and/or oxide, further heating said steel material to a temperature within the austenitic region of from the A1 transformation temperature to the austenitic grain coarsening temperature to effect austenitizing to form a non-mixed grain structure.
 3. The method of claim 2, wherein said steel material is hot worked subsequent to the first heating step and prior to cooling in the 900* - 500*C range.
 4. The method of claim 2, wherein cold working is sequentially cycled with intermediate heat treating, between a first cold working operation and each succeeding cold working operation, said intermediate heat treatment being below the temperature of the A1 transformation point.
 5. The method of claim 3, wherein cold working is sequentially cycled with intermediate heat treating, between a first cold working operation and each succeeding cold working operation, said intermediate heat treatment being below the temperature of the A1 transformation point.
 6. The method of claim 2, wherein said steel is cold worked by machining.
 7. The method of claim 3, wherein said steel is cold worked by machining.
 8. The method of claim 1, wherein after recrystallization said steel material is subjected to carburization treatment in the austenitic region. 